Genetic variability among Schistosoma japonicum isolates from the Philippines, Japan and China revealed by sequence analysis of three mitochondrial genes.

The present study examined sequence variability in the mitochondrial (mt) protein-coding genes cytochrome b (cytb), NADH dehydrogenase subunits 2 and 6 (nad2 and nad6) among 24 isolates of Schistosoma japonicum from different endemic regions in the Philippines, Japan and China. The complete cytb, nad2 and nad6 genes were amplified and sequenced separately from individual schistosome. Sequence variations for isolates from the Philippines were 0-0.5% for cytb, 0-0.6% for nad2, and 0-0.9% for nad6. Variation was 0-0.5%, 0.1-0.8%, 0-0.7% for corresponding genes for schistosome samples from mainland China. For worms in Japan, genetic variations were 0-0.2%, 0.1-0.2% and 0 for the three genes, respectively. Sequence variations were 0-1.0%, 0-1.8% and 0-1.1% for cytb, nad2 and nad6, respectively, among schistosome isolates from different geographical strains in the Philippines, Japan and China. Of the three countries, lowest sequence variations were found between isolates from mainland China and the Philippines and highest were detected between Japan and the Philippines in three mtDNA genes. Phylogenetic analyses based on the combined sequences of cytb, nad2 and nad6 revealed that all isolates in the Philippines clustered together sistered to samples from Yunnan and Zhejiang provinces in China, while isolates from Yamanashi in Japan were in a solitary clade. These results demonstrated the usefulness of the combined three mtDNA sequences for studying genetic diversity and population structure among S. japonicum isolates from the Philippines, China and Japan.